In the context of this specification, the term “physical medium” includes anything to which the term “size” has significance. Size is typically expressed as a number of smaller standard units that equals the size of the physical medium.
For example, the length (size) of a football field (the physical medium) is 100 yards where a “yard” is a “standard unit”
As another example, a barrel of oil (the physical medium) is fifty gallons where “gallon” is a “standard” unit.
As another example, “a physical medium” is a truckload of coal wherein the size is expressed in terms of tons (the standard unit).
An “analog” expression of size is understood to represent a number of smaller standard units that equals the size of the physical medium where all of the standard units have the same size.
A multilevel digital expression of size is understood to be a series of number of groups of standard units, in which the size of each group is different from size of the other groups according to an arithmetic rule. The number of groups of one kind is the “level” of the group.
In a typical physical situation, the actual size of an entity does not change. However, expression of size can be selected according to convenience of the user as being either an analog expression or a multilevel digital expression.
The digital system has evolved over the millennium as a way of expressing size of the physical medium in terms of quantity of standard units.
A size between one and nine digits has been expressed by originating the terms, zero, one, two, three, - - - eight, nine. These terms have been derived to express a size of standard units between 0 and 9.
However, because it is awkward to keep adding new names to numbers greater than 9, the deci-digital system was developed to provide greater convenience.
The decimal system has a base of 10 meaning that the size of a physical medium is a number of standard units where the number, N, is - - -N=n0×100+n1×101+n2×102+ - - -                (n=0, 1, 2, 3 - - - 9 up to a limit determined by the size of the physical medium.)        
The popularity of this system is undoubtedly the result of the convenience for humans having a hand with 10 fingers.
Use of a binary system has exploded with the development of semiconductor components having two energy levels designated 1 and 0 to express size.
The binary system has a base of two digits to which are assigned the values 0 and 1.
The size of the system is expressed as:N=n·20+n″21+n′″22+ - - -
Where n″″ is either 1. or 0
Digital systems are formed on semiconductor chips, packaged in housing provided with pins (terminals). Each chip consists of many gates and memory cells plus a number of input and output attachment points to complex integrated circuits.
An item of information (data, instructions, addresses, etc.) is transmitted between components on wires as a “word” (a group of bits) in binary digital format. Each wire is assigned to transmit a single bit of the word and the value of the bit is either a 1 or zero multiplied by 2n where n is 0, 1, 2, - - -                A major advantage of expressing the data as bits is that the original series of bits values can be preserved indefinitely.        
The resolution of a word depends on the number of bits (word length).
Digital systems are wired with a fixed number of bits per word corresponding to the number of wires of the bus that carries the data.
In digital systems presently on the market, the most common word length is sixteen bits, The digital system executes operations in which a word of one set of sixteen bits is transmitted simultaneously from sixteen terminals of one component over sixteen wires to another set of sixteen terminals of another component
The larger words (sixteen, thirty two or sixty four bits) provide that data is transmitted in parallel 16, or 32 or 64 times as fast respectively, as can be sent by passing the word as single bits serially over one wire.
However, the larger the number of bits per word, the larger will be the number of wires required to send the word in parallel.
Therefore the ideal choice as to the number of bits per word is a compromise between speed of transmission (number of bits transmitted in parallel) and the limit to complexity of the circuit imposed by increasing the number of wires, terminals, etc.